The invention relates to a power supply circuit, provided with a voltage control circuit with negative feedback for applying a rapidly changing high voltage to the anode of a penetron type cathode ray tube for a color display system, and with a deflection voltage correcting unit for obtaining color independent pictures on the screen of the cathode ray tube.
To increase the legibility of a display, a multicolor tube is employed. Owing to the short working distance, it is undesirable to use a shadow-mask tube for this purpose; a penetron tube gives considerably better results. The penetron tube has two different phosphor layers. With a certain anode voltage the electrons excite mainly the first phosphor layer, while with a certain higher anode voltage, the electrons are sufficiently accelerated to penetrate the barrier layer between the two phosphor layers and excite the second phosphor layer. The two anode voltages light up the screen in a certain color; with intermediate anode voltages mixed colors are obtainable. Four-color displays are in common use. In such color displays, the electron beam will hit the screen at a different place for each color with a specific deflection voltage; as the electrons continually acquire a different acceleration by the anode voltages corresponding with the various colors. The deflection voltages should therefore be corrected by a fixed factor for each color. To switch the anode voltage from a first to a second value, a certain time will elapse before the anode voltage settles to its second value and stabilizes. In this switching period, the beam electrons are subjected to a change in acceleration, causing positional errors to be displayed, even if fixed correction factors are applied to the deflection voltages for the individual colors, unless the display is blanked during the switching.
A power supply circuit as set forth in the opening paragraph is known from the German Offenlegungsschrift 26 19 520, in which the deflection voltages are permanently corrected by a fraction of the anode voltage. To improve the long-term stability of the circuit, it is preferable to avoid a direct coupling between the deflection channels and the anode voltage.